This invention relates to an apparatus for monitoring the length of a continuous strip of material and, in particular, to an apparatus for monitoring the length of a continuous strip of material moved by a capstan drive system.
In some film supply systems it is desirous to know the length of film remaining on a supply roll. Furthermore, it is often times desirous to know the length of film on the take-up roll of such systems. Depending on the thickness of the film, the diameter of the film on the supply or take-up roll can be directly translated into the length of the film remaining on the supply or take-up roll. It is desirous to know the length of the film on the supply or take-up roll especially when one wants to meter out a specific length of film and does not want the rilm to run completely off the supply roll before filming is stopped. It is also often times desirous to know in advance if the end of the film is approaching so that the remaining time before the end of the film is encountered can be optimized for changing the film.
Many prior art devices employ arms which ride against the film contained in film rolls or cartridges, which arm subsequently moved a flag or indicator to show how much film remains on the roll or cartridge. Low film indications have also been achieved by detecting holes appearing in the film near the end of the film roll or by placing magnetic foil near the end of the film roll and thereafter detecting the magnetic foil. No prior art devices using a capstan drive system are known which are capable of determing the length of film remaining on a supply or take-up roll without altering the film or being in physical contact therewith.
An apparatus for monitoring the length of a continuous strip of material constructed in accordance with the instant invention includes a supply means for supplying the strip. The supply means includes a rotatable shaft adapted for retaining a roll of the strip and rotatably dispensing a predetermined length of the strip during a given rotation. A capstan drive means receives the dispensed portion of the strip from the shaft and incrementally moves the strip the predetermined length in steps. Monitoring means monitors the length of the strip remaining on the shaft after the rotation by measuring the angular displacement of the shaft after the rotation by measuring the predetermined length of the dispensed portion and by correlating the angular displacement and the predetermined length.